Apparatus for deploying a load to an underwater target position with enhanced accuracy and a method to control such apparatus

ABSTRACT

Apparatus ( 50 ) for deploying an object to an underwater target position, the apparatus being provided with a beacon to transmit acoustic rays, a plurality of thrusters ( 56 ( i ), i=1, 2, . . . I, I being an integer) to control positioning of the apparatus with respect to the underwater target position, and a sound velocity meter to measure velocity of sound in a fluid surrounding the apparatus.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for deploying an object toan underwater target position, the apparatus being provided with abeacon to transmit acoustic rays and a plurality of thrusters to controlpositioning of the apparatus with respect to the underwater targetposition.

Such an apparatus is known from WO 99/61307.

The prior art apparatus is used for deploying and/or recovering loads upto 1000 tons or more on the seabed at great depths, for instance, up to3,000 meter or more. During deployment, the apparatus is controlled bycontrolling equipment on board of a vessel floating on the sea surface.The controlling equipment needs to know the exact location of theapparatus as accurate as possible. To that end, the beacon on board ofthe apparatus transmits acoustic rays through the sea water to thevessel. An appropriate acoustic receiver receives these acoustic raysand converts them into electrical signals used to calculate the positionof the apparatus with respect to the vessel.

However, it is found that with increasing depth of the apparatus belowthe sea water the accuracy of the location measurement decreases due tobending of the acoustic rays in the sea water.

The object of the invention is therefore to further enhance the accuracyof the location measurement of such an apparatus during use in sea wateror any other fluid. Moreover, such location measurement is neededon-line (real-time).

SUMMARY OF THE INVENTION

To obtain this object, the apparatus as defined at the outset ischaracterized in that it is provided with a sound velocity meter tomeasure velocity of sound in a fluid surrounding said apparatus. Thus,the velocity of sound at a certain location in the fluid can becontinuously measured and used to update a sound velocity profile, i.e.,data as to the sound velocity as a function of depth in the fluid. Fromthese data, local bending of the acoustic rays can be determined on-line(real-time). So far, such on-line determination has not been possible.This allows corrections of location measurements in real-time.

In a preferred embodiment, the thrusters comprise a first set ofthrusters arranged to provide a torque control function and a second setof thrusters arranged to provide at least a translation function, eachthruster of the second set of thirsters being provided with a rotaryactuator.

This is a very advantageous embodiment. Only two thrusters are necessaryto prevent any undesired rotation of the apparatus attached to the loadduring deployment thus avoiding all problems related to twisting andturning of hoist wire the load, as already explained in WO 99/61307.Moreover, only two rotatable truss are needed to control positioning ofthe apparatus with its load attached to it to the desired horizontalcoordinates. Thus, prior to lowering the load with the apparatus theapparatus can move the load to the desired horizontal coordinates andwhen these coordinates have been reached the hoist wire(s) can lower theload to its desired location on the seabed while the thrusters keep theload on the desired coordinates and prevent any undesired rotation ofthe load. Only when the desired target position on the seabed is reacheda possible rotation of the load to a desired orientation need be carriedout by the thrusters dedicated to the torque control.

It is observed that rotatable thrusters on an underwater apparatus fordeploying loads to a desired position are known from U.S. Pat. No.5,898,746.

The apparatus is preferably provided with load cells to measure weightof the load attached to the apparatus. When the load is put on theseabed by this weight suddenly decreases. Thus, a signal indicating thatthe weight of the load suddenly decreases can be used to determine whenthe apparatus may be detached from the load.

The invention also relates to a processing arrangement arranged to drivean apparatus for deploying an object to an underwater target position,the apparatus being provided with a beacon to transmit acoustic rays, aplurality of thrusters to control positioning of the apparatus withrespect to the underwater target position, and a sound velocity meter tomeasure velocity of sound in a fluid surrounding the apparatus, theprocessing arrangement being provided with an acoustic receiver toreceive the acoustic rays, the processing arrangement is arranged to usedata derived from the acoustic rays in a calculation to determine theposition of the apparatus characterized in that the processingarrangement is armed to receive online sound velocity meter data fromthe sound velocity meter to determine a sound velocity profile in thefluid and to calculate from the sound velocity profile bending of theacoustic rays transmitted by the apparatus through the fluid and to usethis in the calculation to determine the position of the apparatus inreal-time.

Such a processing arrangement is able to control driving of saidapparatus to a desired location in a desired orientation with very highaccuracy, even at great depth under water. While the apparatus with itsload is lowered, the processing arrangement constantly receives soundvelocity data and determines a sound velocity profile comprising soundvelocity data from the water surface to the depth of the apparatus. Theprocessing arrangement uses these data to determine acoustic ray bendingas a function of the depth in the water and thus to correct any positioncalculation of the apparatus.

Such a processing arrangement may be on board of a vessel floating onthe water surface. However, it is to be understood that part of thefunctionality of determining the sound velocity profile and calculatingthe acoustic ray bending may be carried out by one or more processorslocated elsewhere, even on board of the apparatus itself.

Preferably, a further sound velocity meter is provided just below thewater surface to provide actual data regarding any ray bending in thewater surface layers and thus to further correct any positioncalculation of the apparatus.

Reception of the acoustic rays transmitted by the apparatus ispreferably performed by an acoustic array attached to the hull of thevessel.

In a very preferred embodiment, the vessel, the acoustic array and theapparatus are all provided with a distinct gyrocompass measuringrespective heaves, rolls and pitches. Output data from these gyrocompassare used to further increase accuracy of the position measurement of theapparatus.

The invention also relates to a system comprising such a vessel and anapparatus together.

The invention also relates to a method of driving an apparatus fordeploying an object to an underwater target position, the apparatusbeing provided with a beacon to transmit acoustic rays, a plurality ofthrusters to control positioning of the apparatus with respect to theunderwater target position, and a sound velocity meter to measurevelocity of sound in a fluid surrounding the apparatus, the methodcomprising the steps of:

receiving the acoustic rays,

using data derived from the acoustic rays in a calculation to determinethe position of the apparatus

characterized by the steps of:

receiving sound velocity meter data from the sound velocity meter anddetermining a sound velocity profile in the fluid, and

calculating from the sound velocity profile bending of the acoustic raystransmitted by the apparatus through the fluid and to use this in thecalculation to determine the position of the apparatus.

This method may be entirely controlled by a suitable computer programafter being loaded by the processing arrangement Therefore, theinvention also relates to a computer program product comprising data andinstructions that after being loaded by a processing arrangementprovides said arrangement with the capacity to carry out a method asdefined above.

Also a data carrier provided with such a computer program product isclaimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, the invention will be explained in detail with reference beingmade to the drawings. The drawings are only intended to illustrate theinvention and not to limit its scope which is only defined by theappended claims.

FIG. 1 shows a schematic overview of a FPSO (floating, production,storage and offloading system) dedicated to offshore petrochemicalrecoveries.

FIG. 2 shows a crane vessel according to the prior art and displaying aload rigged to the crane block with relatively long wire ropes wherebyit is possible to see that the control of the load is virtuallyimpossible at great depth.

FIG. 3 shows a crane vessel and an underwater system for deployingand/or recovering a load to and/or from the seabed according to theprior art.

FIG. 4 shows a detailed overview of a possible embodiment of theunderwater system.

FIG. 4a shows a detailed overview of one of the rotatable thrusters.

FIG. 5 shows the underwater system viewed from above.

FIGS. 6a and 6 b schematically show the underside of the main modulewith some detectors.

FIG. 7a shows a schematic block diagram of the electronic equipment onboard of the vessel.

FIG. 7b shows a schematic block diagram of the electronic equipmentrelated to an acoustic array and related to the underwater system.

FIG. 8 shows the definition of three different coordinate systems usedduring driving the underwater system to its target position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, the layout presents a FPSO 1 with swivelproduction stack 11 from which risers 2 depart, said risers connectingto their riser bases 3 at the seabed 4. During production lifetime, itis paramount for the FPSO 1 to remain within an allowable dynamicexcursion range and therefor the FPSO 1 is moored to the seabed 4 bymeans of mooring legs 5 which are held by anchors 6, or alternatively bypiles.

Exploitation of oil or gas by means of a production vessel 1, requiresthat several relatively heavy objects be positioned at the seabed 4 witha high accuracy.

To secure an appropriate and safe anchoring by means of the mooring legs5, it is required that these mooring legs 5 have approximately the samelength. In practice for this application anchors can be used with aweight of 50 ton and more, which are placed at the seabed 4 with anaccuracy to within several meters. Moreover not only is the anchor 6itself very heavy, but the mooring leg attached to the anchor 6 has aweight that equals several times the weight of the anchor 6 itself.

Also for other objects like the “templates”, “gravity riser bases”,“production manifolds” etceteras applies that these objects have to beput on the seabed 4 with relatively high accuracy.

The objects that are shown in FIG. 1 that are required for exploitingthe oil and gas at sea and that have to be put on a seabed, are not onlyvery heavy, but very expensive as well.

FIG. 2 shows a vessel 20, according to the prior art, having hoistingmeans thereon, like a crane 21. The crane 21 is provided with a hoistingwire 22, by means of which an object or a load 4 can be put on theseabed 5. In order to position the load 23 it is necessary to move thesurface support together with the crane 21.

The result will be that, at one given time, the load 23 inertia will beovercome but due to the load 23 acceleration, an uncontrollablesituation will occur, whereby the target area will be overshot. Becauseof the fact that the hoisting wire 22 and the load 4 are susceptible toinfluences like the sea current, the load 23 will not move straightdownward, when the hoisting wire 22 is being lowered. Also the heave,roll and pitch of the vessel 20 will have a negative influence on theaccuracy that can be achieved.

FIG. 3 shows a crane vessel 40 provided with an underwater apparatus orsystem 50 for deploying a load 43 on the seabed 4. The vessel 40comprises first hoist means, for example a winch 41, provided with afirst hoist wire 42. By means of this hoist wire 42 the load 43, forinstance a template can be deployed and placed at the bottom of the sea.

As mentioned above, the exploitation of oil and gas fields using afloating production platform requires that several heavy objects must beplaced at the seabed 4, moreover, these objects have to be placed on theseabed 4 with a very high accuracy. Because of the fact that nowadaysthe exploitation has to be done at increasing depths up to 3000 m andmore, achieving the required accuracy is getting harder. E.g., one ofthe problems to be solved is the possible rotation of the load 43carried by hoist wire 42.

In order to control the position of the load 43 when deploying it and inorder to be able to position the load 43 on the seabed 4 within therequired accuracy, the apparatus or system 50 has been secured to thelifting wire 42. A preferred embodiment of the system 50 will bedescribed with reference to FIGS. 4, 5, 6 a and 6 b.

The system 50 may engage the end of the lifting wire 42. Alternatively,the system 50 may directly engage the load 43 itself. The system 50comprises a first or main-module 51, provided with drive means such asthrusters 56(i), i=1, 2, 3, . . . I, I being an integer (FIGS. 4 and 5).The system further comprises a second or counter module 52. Thiscounter-module 52 is also provided with thruster 56(i). In use thethrusters of the main-module 51 and of the counter-module 52 will bepositioned at opposite sides of the lifting wire 42.

The system 50 is coupled to the vessel 40 by means of a second liftingwire 45, which can be operated using second hoist means, for instance asecond winch 44. The second hoist wire 45 is, for instance, setoverboard by means of an A-frame 49. The second winch 44 and the secondhoist wire 45 will be normally lighter than the first hoist means 48 andthe primary hoist wire 42, respectively. The system 50 is furtherconnected to the vessel 40 by means of an umbilical 46. This umbilical46 can be attached to the hoist wire 45 or can be lowered from atertiary winch 47 separately. The electricity wiring for providing powerto the system 50, as well as electrical wiring or optical fibers are forinstance accommodated in the umbilical. In the system 50 usually meansare provided to convert the electrical power into hydraulic power. Thehydraulic power consequently will be used for controlling i.a. thethrusters 56(i) and auxiliary tooling amenities.

Since lately the work is being done at an increasing depths, twistingand turning of the loads 43 and long hoist wires 42 is becoming a biggerproblem still. Since heavy loads 43 are attached at the underside of thehoist wire 42, such twisting and turning can impel a relatively largewear on the hoist wires, so severe damage can occur at the hoist wires.This wear can be so severe that a hoist wire 42 will break and the load43 will be lost. Another problem is that because of enormous twists inthe wires the wires at the vessel can run out of the sheaves.

Because of the fact that the thrusters 56(i) of the main-module 51 andof the counter-module 52, respectively, are positioned at opposite sidesof the lifting wire 42, a counter-torque can be exerted at the hoistwire 42 in both directions. In this way by means of the system ananti-twist device is formed. In order to improve the abilities of thisanti-twist device, preferably, the distance between the main-module 51and the counter-module 52 can be altered.

FIG. 4 shows a detailed overview of a possible embodiment of the system50 for deploying a load 43 on the seabed 4. FIG. 5 shows the systemaccording to FIG. 4, from above.

The system 50 comprises the main-module 51, the counter-module 52 and anarm 53. The arm 53 can be detached from the main-module 51. That meansthat the main-module 51 can also be used separately, as a modularsystem. The arm 53 is provided with a recess 54. On opposite sides ofthis recess 54 two jacks 57, 58 are provided, at least one of which canbe moved relative to the other. In between the end surfaces of thesejacks 57, 58 an object, such as a crane-block of load 43, can beclamped. In order to improve the contact between the jacks 57, 58 andthe object, the respective ends of the jacks are accommodated withclamping shoes lined with a friction element, from a high frictionmaterial such as dedicated rubber.

In use, the thrusters 56(i) can be used to position the system 50relative to a target area on the seabed 4. The thrusters 56(i) can beactuate from a first position mainly inside the system 50, to a positionin which the thrusters projects out of the system 50. The two upperthrusters 56(2), 56(3) are rotatable with respect to the underwatersystem 50. They are, for instance, installed on respective rotaryactuators 65(1), 65(2). The purpose thereof will be explained later.Thruster 56(2) has been shown on an enlarged scale in FIG. 4a.

In FIG. 5 it is shown that there are two positions 61, 62 on top of themain-module 51 to connect the main module to the second lifting wire 45and/or to the umbilical 46. When the main-module 51 is used separatelyposition 61 can be used. The main-module 61 will be balanced when themodule 61 is deployed, both in the air and underwater.

When the system 50 is used, the connection between the vessel 40 and thesystem 50 will be fixed in position 62 in order to keep the system inbalance, both in the air and underwater. To improve the balance of thesystem, an auxiliary counterweight 55 can be secured to the system 50.

In use, the apparatus 50 will not have any buoyancy. In order to improvethe movability of the system under water, the arm 53 is provided withholes 59, in order to avoid structural damage due to an increasingpressure while being lowered and to ensure quick drainage during therecovery phase.

As mentioned above, it is advantageous when the counter-module 52 can bemoved relative to the main-module 51. This can be accomplished by usingjacks 64 a.

The module 51 comprises an outer frame and an inner frame (both notshown). The inner frame preferably is cylinder-shaped. By connecting theouter frame to the inner frame, a very strong construction can beaccomplished. The strength of the construction is necessary in order toavoid premature fatigue in the system.

The module 51 is, for instance, partly made of high-tensile steel andthereby designed to be used as integral part of either the first 42 orsecond hoist wire 45. This means that the top side of the module 51 willbe connected to a first part of the hoist wire 45, and that theunderside of he module 51 will be connected to a second part of thehoist wire 45, or the underside of the module 51 will be attacheddirectly to the load. In this way the load on the hoist wire will betransferred through the module 51.

As mentioned before, the module 51 is provided with a thruster drive 270for converting electrical power, delivered through the umbilical 46,into hydraulic power. This thruster drive 270 may comprise motors, apump, a manifold and a hydraulic reservoir. Such converting means areknown to persons skilled in the art and need no further explanationhere. In order to communicate relevant data as to its position, bothabsolute and relative to other objects, to the control system and/or anoperator on board of the vessel 40, the module 51 further comprisessensor means and control means that will be explained in detail below.The module 51 is equipped with a sensor junction box. Moreover, themodule 51 comprises light-sources 87, a gyrocompass 256 including heave,roll and pitch sensors, a pan and tilt color camera 97, a USBL responder255 including a digiquartz depth sensor 253, a sound velocity meter 258,and a sonardyne mini Rovnav 264. At the underside of the module 51 aremounted on several platforms light sources 94, a pan and S.I.T. camera93, an altimeter 262, a Doppler log unit 266, and a dual head scanningsonar 260. They are installed there to have only clear sea water belowthem, in use. They are schematically shown in FIGS. 6a and 6 b. It is tobe understood that they may be located elsewhere, e.g., at the undersideof module 52. Moreover, load cells 268 are part of the system 51. Allthese components are schematically indicated in FIG. 7b.

As mentioned above, the use of high resolution sonar equipment 260together with a distance log, measured by Doppler log unit 266, isimportant to achieve the required accuracy, once the load has reachedits intended depth. The sonar equipment 260 will be used to determinethe position with respect to at least one object positioned at theseabed. Using the distance log, it will then be possible to dissociatethe positioning activities from the surface support, as well as from anyother acoustic transponder devices such as LBL (Long Base Line) arrays(or other, e.g., USBL), while accuracy in the order of centimeters willbe achieved within a large radius.

FIG. 7a shows the electronic equipment 200 installed on the vessel 40,whereas FIG. 7b shows deployable acoustic array 250 with velocity meter248 and a gyro compass 252. FIG. 7b also shows underwater electronicequipment 249 installed on the underwater system 50.

The equipment shown in FIG. 7a comprises four processors: a navigationprocessor 202, acoustic processor 224, a sonar control processor 236,and a thruster control processor 240. The navigation processor 202 isinterfaced to the other three processors 224, 236, 240 for mutualcommunications and complementary.

The navigation processor 202 is also interfaced to a surface positioningequipment DGPS (Differential Global Positioning System) 204, a vesselgyrocompass 206, four display units 208, 210, 212, 214, a printer unit218, a keyboard 220, a mouse 222, and a fiber optic (de)multiplexer unit244. If necessary, a video splitter 216 may be provided to transmit oneSVGA signal output of the navigation processor 202 to two or moredisplay units. In FIG. 7a, display units 212, 214 are connected to thenavigation processor 202 via video splitter 216.

The fiber optic (de)multiplexer unit 244 is also connected to theacoustic processor 224, the sonar control processor 236, and thethruster control processor 240.

The acoustic processor 224 is connected to a command and control unit226 which is connected to a keyboard 230, a mouse 232 and a display unit228, all together forming a USBL surface unit 234.

The acoustic processor 224 is connected to deployable acoustic array 250with motion sensor unit 252 and velocity meter 248. In use, the acousticarray 250 is, preferably, mounted 2.5 meters below the keel of vessel40.

The fiber optic (de)multiplexer unit 244 is connected to a further fiberoptic (de)multiplexer 246 installed on the underwater system 50. Anoptical fiber interconnecting both fiber optic (de)multiplexers 244, 246is preferably accommodated in umbilical 46 (FIG. 3).

The sonar control processor 236 is connected to a display unit 238. Thethruster control processor 240 is connected to a display unit 242.

The underwater equipment 249 is shown in FIG. 7b in the form of a blockdiagram. The USBL responder 255 with digiquartz depth sensor 253, agyrocompass with motion sensors 256, (removable) sound velocity meter258, a dual head scanning sonar 260, altimeter 262, sonardyne miniRovnav 264, Doppler log 266, load cells 268, and thruster drive control270 are all connected to the fiber optic (de)multiplexer 246.

Moreover, FIG. 7b shows two beacons 272, 274 that can be installed onthe seabed or on the load to be deployed (or on other structures alreadyon the seabed). These beacons 272, 274 can, e.g., be interrogated bymeans of the sonardyne mini Rovnav 264 (or equivalent equipment) totransmit acoustic signals back to the system 50 that can be used by thesystem 50 itself to determine and measure distances and orientationsrelative to these beacons. Such an acoustic telemetry link results invery high precision relative position measurements. The number of suchbeacons is not limited to the two shown in FIG. 7b.

Functionality

The functions of the components shown in FIGS. 7a and 7 b are thefollowing.

The navigation processor 202 is collecting the surface positioningequipment data (DGPS receivers, DGPS corrections, vessel's gyrocompassand vessel's motion sensors 204 and 206), in order to calculate anddisplay the vessel's attitude and its fixed offsets.

Via the fiber optic (de)multiplexers 244 and 246, the navigationprocessor 202 sends different settings to the navigation instruments ofthe system 50, i.e., Doppler log 266, altimeter 262, and gyrocompass andmotion sensors 256. After setting up, it receives the data from thoseinstruments, as well as, via the acoustic processor 224, therange/bearing and depth data of the system 50 to calculate and todisplay the attitudes and absolute coordinates of the system 50.

An integrated software in the navigation processor 202 has beendeveloped, including a dynamic positioning controller software able towork in manual or automode to decide the intended heading of the system50 and to select between many way points and to carry out the intendedpositioning. Moreover, the operator on board of the vessel can inputoffsets to the selected way point, the offsets being input with XYcoordinates relative to the heading of the system 50. There is anotherpossibility to select several other types of sub-sea positioning devicesvia an arrangement of specifically designed windows on the screens(electronic pages) of the display units 208-214, to stabilize and filterthe position. To ensure that the operator has as many tools as possibleto get the optimal result, there is an other part in the softwareshowing different status of the sub-sea instruments in use for thecalculation of the position of the system 50 on-line (real-time).

Embarked gyrocompass 256 including heave, roll and pitch sensors 88 onboard of the system 50 provides data as to the exact attitudes of boththe system 50 and the load 43 to be installed on the sea bed. At thesurface of the sea, in a control van, operators are able to check thoseattitudes on-line (real-time), during descent but also once the load 43is laying on the sea bed for final verification.

The vessel gyrocompass 206, as well as the gyrocompass with motionsensors 252 installed on the acoustic array 250 that could be used forthe same functions, is transmitting the vessel's heading to thenavigation processor 202. The navigation processor 202 will use thisvessel's heading to calculate different offsets.

The display units 208, 210, 212, and 214, respectively, are arranged todisplay navigation settings, a view of the sea bed, a view of thesurface, in the control van for the operators and another one on thevessel bridge for the marine department operators.

The USBL command and control unit 226 consists of a personal computerproviding control and configuration of the system and displaying theman-machine-interface for operator control.

The acoustic processor 224, preferably, consists of one VME rack whichperforms correlation process on received signals, corrections tobathy-celerimetry and vessel's attitude. Moreover, it calculatescoordinates of any beacon used. The acoustic processor 224 is linked tothe navigation processor 202 through Eternet.

The acoustic array 250 includes means for transmission and reception.The acoustic array 250 can be used as a transducer to acousticallycommunicate with one or more beacons. Such a transducer mode isadvantageous when the umbilical 46 fails and is unable to transmitinterrogation signals down to the system 50. Then, acousticinterrogation signals can be transmitted down by the transducer directlythrough the sea water. In all other cases, the acoustic array 250 willbe used in a reception mode. Reception is done with two orthogonalreception bases which measure distances and bearing angles of beaconsrelative to the acoustic array 250. Each reception base includes twotransducers. Each received signal is amplified, filtered and transferredto the acoustic processor 224 for digital signal processing.

The sound velocity meter 248 installed on the acoustic array 250 isupdating in real-time the critical and unsettled sound velocity profilesituated just underneath the vessel 40. This is of great importancesince turbulences of the sea water appear to be very heavy in theselayers just underneath the vessel 40.

The gyrocompass 252 is preferably used as motion sensor unittransmitting the acoustic array attitude to the acoustic processor 224in order to rectify data as to the position of the system 50 sub-sea.

In a preferred embodiment, the beacon 254 is working in a responder modeand has the following characteristics:

the triggering interrogation signal generated by the acoustic processor224 is not acoustic but electrical and is transmitted to the beacon 254through the cable link between the vessel 40 and the system 50;

interrogation frequencies are remotely controlled by an operator throughthe man-machine-interface.

As indicated above, the beacon 254 can also be used in a transpondermode. Then, the beacon 254 is triggered by a surface acoustic signaltransmitted by the acoustic array 250 and then delivers acoustic replysignals to the acoustic array 250 through a coded acoustic signal.

The digiquartz depth sensor 253 included in the beacon 254 allowstransmitting very accurate depth data of the system 50 to the acousticprocessor 224. The acoustic processor 224 uses these data to improve thecalculation of the sub-sea positioning of the system 50 and its load 43.

The sound velocity meter 258, mounted on the underwater system 50, istransmitting data as to the velocity of sound in sea water at the depthof the underwater system 50 to the acoustic processor 224 during descentand recovery. The sound velocity data is used to update calculated soundvelocity profiles in the sea water as a function of depth in real-timeand to calculate acoustic ray bending from these profiles as function ofdepth in the sea water and thus to correct calculations of the sub-seaposition of the system 50.

The dual head scanning sonar 260 is used to measure ranges and bearingsof the system 50 to any man-made or natural target on the seabed and tooutput corresponding data as digital values to the navigation processor202. The positions of such man-made or natural targets can either bepredefined or the navigation system can allocate coordinates to each ofthe selected objects. After the objects have been given coordinates,they can be used as navigation references in a local coordinate system.This results in an accuracy of 0.1 meter in relative coordinates.

The altimeter 262 mounted on the system 50 is measuring the verticaldistance of the underwater system 50 to the seabed and transmits outputmeasuring data to the acoustic processor 224.

The Doppler log unit 266 provides data as to the value and direction ofthe sea water current at the depth of the underwater system 50. Thesedata are used in two ways.

First of all, the data received from the Doppler log unit 266 and thegyrocompass with motion sensor 256 is used by the acoustic processor 224to smooth on-line (real-time) the random noise related to using USBL. Toobtain such a smoothing a filter is used, e.g., a Kalman filter, aSalomonsen filter, a Salomonsen light filter, or any other suitablefilter in the main processor unit 224. Such filters are known to personsskilled in the art. A brief summary can be found in appendix A.

Secondly, the output data of the Doppler log unit 266 regarding currentstrength, current direction, together wit data regarding present andintended heading of the underwater system 50 are transmitted to thethruster control processor 240 via the navigation processor 202. Basedon the intended direction the thruster drive control 270 will beautomatically controlled. Manual control may also be provided for.

In a very advantageous embodiment the Doppler log unit 266 (or any othersuitable sensor) is used to measure temperature and/or salinity of thesea water surrounding the system 50. Data as to local temperature and/orsalinity is transmitted to the navigation processor 202 that calculatesand updates temperature and/or salinity profiles as a function of depthin the sea water. These data are also used to determine acoustic raybending through the sea water and, thus, to correct calculations of theposition of the system 50.

The sonardyne mini Rovnav 264 is optional and may be used to providerelative position of the system 50 to local beacons on the seabed asexplained above. For instance, a Long Base Line (LBL) array may alreadybe installed on the seabed and used for that purpose.

The load cells 268 are used to measure the weight of the load 43 asengaged by the underwater system 50. When this weight decreases this isan indication that the load is now placed on the seabed (or other targetposition) and that the system 50 may be detached from the load 43.Output data from the load cells is transmitted to the navigationprocessor 202 through the (de)multiplexers 244, 246.

The thruster drive control 270 is used to drive the thrusters 56(i) inorder to drive the underwater system 50 to the desired position as willbe explained in detail below.

In FIG. 7a, four different processors 202, 224, 236 and 240 are shown tocarry out the functionality of the system according to the invention.However, it is to be understood that the functionality of the systemcan, alternatively, be carried out by any other suitable number ofcooperating processors, including one main frame computer, either inparallel or master slave arrangement. Even remotely located processorsmay be used. There may be provided a processor on board of theunderwater system 50 for performing some of the functions.

The processors may have not shown memory components including harddisks, Read Only Memory's (ROM), Electrically Erasable Programmable ReadOnly Memory's (EEPROM) and Random Access Memory's (RAM), etc. Not all ofthese memory types need necessarily be provided.

Instead of or in addition to the keyboards 220, 230 and the mice 222,232 other input means known to persons skilled in the art, like touchscreens, may be provided too.

Any communication within the entire arrangement shown may be wireless.

In FIG. 5, the situation is shown that the two upper thrusters 56(2) and56(3) are directed in an other direction than the thrusters 56(1) and56(4). The thrusters 56(2), 56(3) are mounted on rotary actuators 65(1),65(2), which allow the thrusters 56(2), 56(3) to be vectored by turningthem up to 360°. Preferably, the thrusters 56(2), 56(3) can beindependently controlled such that they may be directed each to adifferent direction.

To allow the thruster control processor 240 to accurately position theunderwater system 50, a common coordinate system must be establishedbetween the navigationprocessor 202 and the thruster control processor240. First of all, there is a standard coordinate system used by thenavigation processor 202. However, two other coordinate referencesystems are preferably established for the underwater system 50.

FIG. 8 shows the three different coordinate systems. The coordinatesystem related to the navigation processor 202 is indicated with“navigation grid”. This coordinate system uses this “navigation grid”direction and its normal.

The thrusters 56(2), 56(3) are controlled to provide a driving force ina direction termed “thruster mean direction”. This direction togetherwith its normal defines the second coordinate system.

The third coordinate system is defined relative to the “systemdirection” which is defined as the direction perpendicular to a lineinterconnecting the thrusters 56(1), 56(4).

Now, an error in the path followed by the underwater system 50 can bedefined in terms of an error vector that can be split into one componentparallel to the thruster mean direction termed the “mean error” and acomponent normal to the thruster mean direction termed “normal meanerror”. Appropriate sensors on the underwater system 50 will provide thenavigation processor 202 with the thruster mean direction and systemdirection. From these data the navigation processor 202 will create agrid as shown in FIG. 8.

The error is defined as the desired position DP minus the systemposition TP such that a vector RΦ_(EN) is generated relative to thenavigation grid reference, i.e.:

DP−TP=RΦ _(EN)

Moreover:

Φ_(TN) is the system orientation minus the navigation grid orientation,

Φ_(MT) is the mean thruster orientation minus the system orientation.

Then:

DP−TP=RΦ _(EM), Φ_(EM)=Φ_(EN)−(Φ_(TN)+Φ_(MT))

Now RΦ_(EM) is known, the mean and the normal to the mean errors can becalculated.

The two thrusters 56(1) and 56(4) are used to counteract the twistingforces applied by the lifting cable 42, equipment drag and therotational moment induced by the vectoring of positioning control. Acontrol loop for the orientation requires that the navigation processor202 is provided with the actual system orientation and the desiredsystem orientation. The actual system orientation is measured by thegyrocompass 256. The desired orientation is manually input by anoperator. From these two orientations the control loop in the navigationprocessor 202 computes an angular distance between the requiredorientation and the actual orientation as well as the direction ofrotation required to move the system 50 accordingly. A simple controlloop controlled by the thruster control processor 240 then adjusts thepower to the thrusters 56(1) and 56(4) to rotate the system 50appropriately.

On power up of the system 50, both thrusters 56(2) and 56(3) will be,preferably, orientated such that the thruster mean direction is directedparallel to the system direction. Then, the thrusters 56(2), 56(3) willbe given a small vector angle deviation from the system direction toassist in positioning the system 50 in two planes. The size of thisvector is, preferably, manually adjustable and may be needed to beconfigured for each different job in dependence on actual seaconditions. Once the thrusters 56(2) and 56(3) have been centered andvectored, a positioning loop can take over control of the system 50.

The positioning loop comprises two more phases.

In the first next phase, which is executed while the system 50 is stillnear the sea surface, the sea current direction will be measured by theDoppler log unit 266. The sea current direction will be transmitted tothe navigation processor 202. Using this direction, the thruster controlprocessor 240 receiving proper commands from the navigation processor202 will drive the rotary actuators 65(1), 65(2) such that the thrustermean direction substantially opposes the sea current direction. Duringthis rotation of the rotary actuators 65(1), 65(2) none of thrusters56(i) is powered. The system direction will be measured by the fiberoptic gyrocompass 256. The depth is constantly measured by thedigiquartz depth sensor 254 and the altitude by the altimeter 262. Themean and normal to the mean errors as calculated in accordance with theequations above will then be used by the positioning loop to apply powerto the thrusters 56(2) and 56(3) to drive the system 50 to the desiredlocation.

During driving the system 50 with load 43 to the desired coordinates bymeans of thrusters 56(2), 56(3) the thrusters 56(1), 56(4) are used tocounteract any rotation of the system 50 with its load 43. This providesfor better control since, especially for heavy loads, rotation movementsmay result in other undesired movements of the load, which may bedifficult to control. When the system 50 with its load is on the desiredcoordinates the load together with the system 50 is lowered by means ofthe hoisting wire 42. During descending the load 43, the load 43 isconstantly controlled by system 50 to keep it on the desired locationwithout any rotation.

In a next phase, the system 50 is for instance approximately 200 m orless from the seabed 4. Then, the Doppler log unit 266 goes into abottom track mode. This changes the operation into a more accurate andfast responding mode for the final approach of the target location onthe seabed 4. Now, the Doppler log unit 266 and the gyrocompass withmotion sensors 256 are used to filter the random noise of the USBL. Oncefiltered, a good read out of the navigation data including an accuratevelocity of the system 50 will make the position control loop bothextremely rapid and stable. A very fine tuned control loop results inwhich control up to some centimeters movement is achieved. Now, thesonar unit 260 and the Doppler log unit 266 are used to provideinformation regarding the surroundings of the target point such that theload 43 can be positioned on the right coordinates and in the rightorientation. Then, a rotation, if necessary, may be applied to the load43 by thrusters 56(1), 56(4) as controlled by thruster control processor240.

Two control loops are provided for the thrusters 56(2), 56(3): a meanerror control loop and a further control loop to reduce the normal meanerror.

The mean error control loop will adjust the power equally to boththrusters 56(2), 56(3) so as to reduce the mean error. As the system 50reaches the target coordinates the driving power to the thrusters 56(2),56(3) will be reduced to such a level that the system 50 is able tomaintain its position in the sea current. In other words, initially, thedriving power was set at a level that was proportional to the meanerror. However, as the system 50 moves closer to the target coordinatesthe control loop will slowly reduce the driving power applied to thethrusters 56(2), 56(3). As the system 50 reaches the target coordinatesan equilibrium will be reached where the driving power to the thrusters56(2), 56(3) counteracts the strength of the sea current. The mean errorcontrol loop provides equal power with equal sign to both thrusters56(2), 56(3).

A further control loop is applied to reduce the normal mean error. Thisfurther control loop adjusts the individual power applied to thethrusters 56(2), 56(3) such that a movement perpendicular to the seacurrent is generated. The further control loop applies equal power ofopposite sign to both thrusters 56(2), 56(3) to this effect. The powerapplied to the thrusters 56(2), 56(3) in order to reduce the normal meanerror, preferably, reduces linearly to zero as the system 50 moves tothe target coordinates. At the point where the normal to the mean errorreaches zero and assuming that the sea current direction has notchanged, the system 50 will exactly be located above the target positionon the sea bed 4 and the thrusters 56(2), 56(3) are powered to keep thesystem 50 on the correct coordinates and to correct for the sea current.

If the sea current direction changes the control loops referred to abovewill be required to adjust the power applied to the thrusters andultimately to change the system direction. As the new current directionacts upon the system 50, the normal mean error will start to increase asthe system 50 is moved from the target coordinates. To overcome thiseffect, the size of the normal mean error will again be controlled toreduce to zero. The system direction is changed such that the seacurrent or natural drift of the system 50 is counteracted.

The direction of rotation of the rotary actuators 65(1), 65(2) will bedefined by the sign of the normal mean error. To reduce the timerequired to slew the rotary actuators 65(1), 65(2) to the requiredposition, an algorithm will be used by the thruster control processor240 to determine the shortest route to the required orientation.

It is envisaged that manual control by means of, for instance, ajoystick (not shown) connected to the navigation processor 202 is alsoarranged.

During the positioning of the system 50 a velocity control is also,preferably, applied. Preferably, the closer is the system 50 to thecoordinates of the target, the slower will be the velocity of the system50. For instance, when the distance between the system 50 and the targetis more than a predetermined first threshold value, the thrusters arecontrolled to provide the system 50 with a maximum velocity. Betweenthis first threshold value and a second threshold value of the distanceto the target coordinates, the second threshold value being lower thanthe first threshold value, a linearly decreasing velocity profile isused. Within a distance smaller than the second threshold value thesystem is kept on a velocity of substantially zero.

USBL Measurement

The USBL measurement principle is based on an accurate phase measurementbetween two transducers. In one embodiment, a combination of short baseline (SBL) and ultra short base line (USBL) is used which enables to usea large distance between transducers without any phase ambiguity. For anUSBL, the accuracy depends on the signal to noise ratio and the distancebetween the transducers (like in an interferometry method). Then, thetrade-off is for frequency which is limited by the range andhydro-dynamic part in terms of dimensions.

Ambiguity is calculated by using an SBL measurement combined withcorrelation data processing. The signal-to-noise ratio is improved byuse of such correlation processing. The following expression defines thegeneral accuracy for a USBL:$\sigma_{\theta} = {K\frac{\lambda}{L\sqrt{\frac{signal}{noise}}\cos \quad \vartheta}}$

where:

σ_(θ): Angular standard deviation

L: transducer distance

λ: wavelength

θ: bearing angle

The expression given above indicates that the accuracy is improved byincreasing the transducer distance L, i.e., by increasing the array.Moreover, a higher frequency results in a better accuracy. Hydrodynamicaspects and phase ambiguity reduce these parameters. Signal-to-noiseratio is increased by using correlation data processing.

To optimize range and accuracy, a frequency of 16 kHz is preferably usedfor phase meter measurements. A correlation process enables to increasethe distance range while keeping a narrow pulse length for multipathdiscrimination.

For ambiguity phase measurements, the system operates in SBL todetermine a range sector and in USBL within the sector to achieve thebest accuracy.

The range may be increased beyond 8000 m by using a rather lowfrequency.

Appendix A

Kalman Filter

The Kalman filter is probably the most well-known technique in theoffshore industry. It gives a fast filtering method based on comparisontowards predicted values, which are calculated on basis of the latesthistory. We will not go into details about Kalman filtering, but referto, e.g., “Kalman Filtering—Theory and Practice”, by M. S. Grewal and A.P. Andrews Prentice Hall (ISBN 0-13-211335-X).

The position track can be combined with the velocity data (Doppler log),each point will be improved on basis of the neighboring points, thedistance in time and the actual speed. The weight between Kalman valueand the velocity improved is decided by the Doppler efficiencycoefficient: higher values will take speed more into consideration.

Advantage: Disadvantage: It's fairly fast Rather ‘un-smooth’ result Canbe improved with speed Not the best combination of speed and position

Simple Filter

The Simple filter runs through all positions, and calculates a smoothcurve giving a minimum squared error, i.e. a kind of Least Square Fitline

Advantage: Disadvantage: It's fast No Doppler-log data is used Theresult is smooth Does not like curved tracks

Salomonsen Filter

The Salomonsen filter, which is named after the Danish mathematicianHans Anton Salomonsen, Professor and phD at University of Aarhus, is ahighly integrated filter. It takes advantage of the short-term stabilityof the Doppler track and combines it with the long-term robustness ofthe position track.

Description

The filter is used in a situation where we have time tacked positiondata along a track as well as Doppler data. The Doppler Data are usuallyvery precise but do not give any information about the absolutepositions. On the other hand the position data are absolute positionsbut they are usually not very precise. The filter combines the two setsof data to produce a precise track with absolute positions. This is doneas follows.

1. The Doppler data are used to construct the shape of the track, i.e. atack formed as a cubic spine.

2. Beginning at the origin (0, 0) and with velocities as defined by theDoppler data

3. Then the position data are used to position the track correctly. Thetrack is translated, rotated, and stretched/compressed linearly to fitthe position data as well as possible using least squares techniques.

4. It will mainly be a translation. However, the other modificationsserve to correct for possible systematic errors in the Doppler data.

The fact that the position data are used only to make the modificationsin 2 means that the position data are subject to considerable averaging.This reduces the uncertainty of the position measurements. Thus, ifthere are many position data the absolute position of the track shouldbe expected to be much more precise than each single positionmeasurement.

H. A. Salomonsen

Mathematical Description

The algorithm is divided into five steps:

Step 1:

Calculate accelerations for each point

1/2hk+1(X 1″+Xk+1″)=Xk+1′−Xk′

Where

hk=tk−tk−1

tk=timestamp for speed measurement

Xk′=speed measurement at tk

Xk″=calculated acceleration at tk

Step 2:

Calculate next position based on acceleration and speed, and previouscalculated position (based on previous speed measurements andaccelerations)

Xk+1=Sqr(hk+1)/6(2Xk″+Xk+1′)+hk+1Xk′+Xk

Where

xk=calculated position at tk (speed timestamp)

Step 3:

Calculate the positions at actual timestamps (using position of firstspeed measurements)

X(t)=1/2hk+1{((hk+1){circumflex over ( )}2(t−tk)+1/3(tk+1−t){circumflexover ( )}3−1/3(hk+1){circumflex over ( )}3)Xk″+1/3(t−tk){circumflex over( )}3Xk+1″}

Where

X(t)=position at time t

Step 4:

Add position of first speed measurements to calculated positions

Step 5:

Move, rotate, stretch of compress calculated positions to best fit ofreal position line

Advantage: Disadvantage: It combines the best of Doppler and It is slowdue to complex matrix positions. Dependent on good Doppler-log Takes alldata in consideration The result is smooth

Salomonsen Light

The light version of Salomonsen filter, which was first introduced inthe NaviBat On-line program, was invented to have a faster solutioncombining the better of two methods.

Due to its on-line nature, it only uses history in deciding to filter apoint. Hence the result will be rougher at the start of line and gettingbetter as it moves along.

Basic Operation

The filter is started with a reset call to initialize the filter. Thereset is made using the first velocity measurement. The filter uses bothvelocity and position data. A cubic spine curve is created using thevelocity records and fitting the positions as good as possible to thiscurve.

Then the filter is reading a position record it is stored for laterprocessing.

When a velocity record is read a ‘knot’is created. Any positions readbetween the previous and the present velocity records (in time) areadjusted to fit the curve.

History

The filter gain parameter, value 0 to 1, controls the influence ofDoppler-log data and history on the current point.

For the value 1 the Doppler-log data and history in the line have thegreater weight. Smaller values are only when there are more positionrecords than valid, velocity records.

Useful values will be in the range 0.9 to 1, e.g. 0.99.

Error Correction

The position and velocity records may be compared with predicted valuesusing previous data limits may be set when to reject data.

Resetting

If there are many erroneous data points there is a risk that the filterlooses track. The operator may reset the filter manually, i.e. kill itshistory (attempts are made to design an auto-reset).

Advantage: Disadvantage: It combines the best of Doppler and positions‘un-smooth’ at the start of It is fast line The overall result is smoothCan handle noisy Doppler data

What is claimed is:
 1. Apparatus (50) for deploying an object (43) to anunderwater target position, the apparatus being provided with a beaconto transmit acoustic rays to a surface vessel for determining a positionof the apparatus and a plurality of thrusters (56(i), i=1,2, . . . I, Ibeing an integer) to control positioning of said apparatus with respectto said underwater target position, wherein the apparatus is providedwith a sound velocity meter (258) to continuously measure velocity ofsound in a fluid surrounding said apparatus during descent and recoveryand to transmit sound velocity data in real-time to the surface vesselfor updating calculated sound velocity profiles in the fluid as afunction of depth in real-time and correcting the determined position ofthe apparatus.
 2. Apparatus according to claim 1, wherein said thrusterscomprise a first set of thrusters (56(1), 56(4)) arranged to provide atorque control function and a second set of thrusters (56(2), 56(3)arranged to provide at least a translation function, each thruster ofsaid second set of thrusters (56(2), 56(3)) being provided with a rotaryactuator (65(1), 65(2)).
 3. Apparatus according to claim 1, wherein saidapparatus is provided with a gyrocompass with motion sensors (256) tosense roll and pitch of the apparatus in use.
 4. Apparatus according toclaim 1, wherein the apparatus is provided with a sonar unit (260) todetermine the position of said apparatus with respect to at least oneobject external to said apparatus.
 5. Apparatus according to claim 4,wherein the apparatus is provided with a Doppler log unit (266) tomeasure current strength of said fluid.
 6. Apparatus according to claim1, comprising load cells (268) to measure weight of a load (43) engagedby the apparatus.
 7. Apparatus according to claim 1, wherein theapparatus is provided with a temperature sensor (266) to measuretemperature in said fluid and to transmit temperature data in real-time.8. Apparatus according to claim 1, wherein the apparatus is providedwith a salinity meter (266) to measure salinity of said fluid and totransmit salinity data in real-time.
 9. A processing arrangementarranged to drive an apparatus (50) for deploying an object (43) to anunderwater target position, the apparatus being provided with a beaconto transmit acoustic rays to a surface vessel for determining a positionof the apparatus, a plurality of thrusters (56(i), i=1,2, . . . I, Ibeing an integer) to control positioning of said apparatus with respectto said underwater target position, and a sound velocity meter (258) tocontinuously measure velocity of sound in a fluid surrounding saidapparatus during descent and recovery and to transmit sound velocitydata in real-time, the processing arrangement being provided with anacoustic receiver (250) to receive said acoustic rays, the processingarrangement is arranged to use data derived from said acoustic rays in acalculation to determine the position of the apparatus, the processingarrangement being arranged to receive on-line sound velocity meter datafrom said sound velocity meter (258) to continuously update a soundvelocity profile in said fluid as a function of depth and to calculatefrom said sound velocity profile bending of said acoustic raystransmitted by the apparatus through the fluid and to use this in theupdated sound velocity profile to correct the calculation to determinethe position of said apparatus in real-time.
 10. Processor arrangementaccording to claim 9, wherein said thrusters of the apparatus comprise afirst set of thrusters (56(1), 56(4)) arranged to provide a torquecontrol function and a second set of thrusters (56(2), 56(3) arranged toprovide at least a translation function, each thruster of said secondset of thrusters (56(2), 56(3)) being provided with a rotary actuator(65(1), 65(2)), and said processing arrangement is arranged to performthe following functions in use: to control application of driving powerto said thrusters of said first set of thrusters (56(1), 56(4)) to keepsaid apparatus in a desired orientation in a first plane defined bydriving forces generated by said thrusters (56(1), 56(4)) of said firstset; to control application of driving power to said thrusters of saidsecond set of thrusters (56(2), 56(3)) and to said rotary actuators(65(1), 65(2)) to move said apparatus in a mean direction and adirection perpendicular to said mean direction to a desired location,said mean direction and said direction parallel to said mean directionbeing in a second plane defined by driving forces generated by saidthrusters (56(1), 56(4)) of said second set.
 11. A processingarrangement according to claim 10, in said apparatus said first andsecond plane not being coincident, the processing arrangement beingarranged to receive first sense signals from a gyrocompass with motionsensors (256) on the apparatus (50) regarding roll and pitch of theapparatus in use.
 12. A processing arrangement according to claim 11,wherein the first sense signals from the gyrocompass with motion sensors(256) are used in the calculation to determine the attitude of theapparatus.
 13. A processing arrangement according to claim 9, theapparatus including a temperature sensor (266), wherein the processingarrangement is arranged to receive temperature data from saidtemperature sensor, to update a temperature profile in said fluid and toassist a correction of determining the position of said apparatus inreal-time.
 14. A processing arrangement according to claim 9, theapparatus including a salinity meter (266), wherein the processingarrangement is arranged to receive salinity data from said salinitymeter, to update a salinity profile in said fluid and to correctdetermining the position of said apparatus in real-time.
 15. A vesselprovided with a processing arrangement according to claim
 9. 16. Avessel according to claim 15, wherein the vessel is provided with anacoustic array (250) attached to a hull of the vessel and an ultra shortbase line surface unit (234) on board of the vessel arranged tocommunicate with said acoustic array (250), the acoustic array (250)being arranged to receive acoustic signals from at least said apparatus(50) and to provide acoustic array output data to said processingarrangement, which is arranged to perform, in real-time, the calculationof the position of at least said apparatus (50) relative to saidacoustic array (250) based on said acoustic array output data.
 17. Avessel according to claim 16, wherein the acoustic array (250) comprisesa sound velocity meter (248) to measure velocity of sound in fluidlayers just below the vessel and to provide sound velocity meter outputdata to said processing arrangement, said processing arrangement beingarranged to correct said calculation of said position of said apparatus(50) based on said sound velocity meter output data in real-time.
 18. Avessel according to claim 16, wherein the acoustic array (250) comprisesan acoustic array gyrocompass (252) to measure heave, roll and pitch ofthe acoustic array (250) and to provide acoustic array gyrocompassoutput data to said processing arrangement, the processing arrangementbeing arranged to correct said calculation of said position of saidapparatus (50) based on said acoustic array gyrocompass output data inreal-time.
 19. A vessel according to claim 15, wherein the vesselcomprises a vessel gyrocompass (206) to measure heave, roll and pitch ofthe vessel and to provide vessel gyrocompass output data to saidprocessing arrangement, the processing arrangement being arranged tocorrect said calculation of said position of said apparatus (50) basedon said vessel gyrocompass output data in real-time.
 20. A systemcomprising a vessel according to claim 15, the apparatus and theprocessing arrangement being arranged to communicate with one another.21. A system according to claim 20, wherein the apparatus and theprocessing arrangement are coupled via fiber optic (de)multiplexers(244, 246) interconnected by an optical fiber.
 22. A method of drivingan apparatus (50) for deploying an object (43) to an underwater targetposition, the apparatus being provided with a beacon to transmitacoustic rays to a surface vessel for determining a position of theapparatus, a plurality of thrusters (56(i), i=1,2, . . . I, I being aninteger) to control positioning of said apparatus with respect to saidunderwater target position, and a sound velocity meter (258) tocontinuously measure velocity of sound in a fluid surrounding saidapparatus during descent and recovery and to transmit sound velocitydata in real-time, the method comprising the steps of: receiving saidacoustic rays from the beacon, using data derived from said acousticrays in a calculation to determine the position of the apparatus,receiving sound velocity meter data from said sound velocity meter (258)and continuously updating a sound velocity profile in said fluid as afunction of depth, and calculating from said sound velocity profilebending of said acoustic rays transmitted by the apparatus through thefluid and using the updated sound velocity profile to correctcalculation to determine the position of said apparatus.
 23. A computerprogram product comprising data and instructions that after being loadedby a processing arrangement provides said arrangement with the capacityto carry out a method according to claim
 22. 24. A data carrier providedwith a computer program product according to claim
 23. 25. Apparatusaccording to claim 1, further comprising a second sound velocity meter(248) to measure velocity of sound in fluid layers just below the vesseland to provide sound velocity meter output data for correcting theposition of the apparatus based on said sound velocity meter output datain real-time.